The disclosure relates to a hydrostatic transmission, a traction drive having the transmission, and a method for regulating a drive output torque of the transmission.
The laid-open specification DE 10 2012 106 370 A1 has disclosed a hydrostatic drive with a hydrostatic transmission with an adjustable hydraulic pump and an adjustable hydraulic motor, which are arranged in a closed hydraulic circuit. In order to regulate a working pressure of the circuit within admissible limits, and in order to protect a drive machine (diesel engine) against overloading owing to excessively high torques, the pressure is regulated. This is performed without a pressure sensor arrangement. Instead, the working pressure is determined in model-based fashion from a volume flow balance of the hydraulic pump and of the hydraulic motor, more precisely from a volume flow difference between the two machines. Traction, neutral or braking operation of the drive machine is likewise inferred from the volume flow difference. A different setpoint working pressure of the circuit arises depending on the operating mode. Said setpoint working pressure corresponds to a setpoint volume flow difference. The two, the balanced volume flow difference and the setpoint volume flow difference, are compared, and the resulting deviation is input as a corrective factor into the actuation of the displacement volumes of the hydraulic pump and of the hydraulic motor. Here, the control signals of the adjustment devices of the two machines are used as actual values of the displacement volumes, such as are required for the balancing.
A disadvantage of this is that even a very small error in the controlled displacement volume, assumed as actual value, of the hydraulic pump or of the hydraulic motor can lead to considerable deviations in the working pressure of the circuit. At the same time, however, the dynamics of the pressure change are very high. The described pressure regulation by way of the variation of the displacement volumes furthermore requires a very short cycle time, of approximately one millisecond, for the actuation. In practice, it has been found that the displacement volume of the hydraulic motor must be detected by way of a sensor, because errors in the displacement volume of the hydraulic motor have a severe effect on the volume flow balance.
The European patent application EP 2 767 739 A1 presents a power-split hydrostatic drive in a closed circuit with an adjustable hydraulic pump and a hydraulic motor. Here, owing to the type of construction of the hydraulic pump, the working pressure of the circuit counteracts an enlargement of a displacement volume of the hydraulic pump. In a manner dependent on definable traction ranges of the drive, for example slow working travel or fast service travel, a relatively low (working travel) or relatively high (service travel) control or setting pressure is provided to the hydraulic pump for adjustment of its displacement volume. The control pressure, which counteracts the working pressure, is thus a measure for the working pressure beyond which the displacement volume of the hydraulic pump is reduced in size. Thus, by way of the setting of the control pressure, it is possible for the working pressure, and therefore a torque and, at a given rotational speed, a power, to be limited.
By contrast, it is the object of the disclosure to provide a hydrostatic transmission with a more precisely controllable drive output torque. The disclosure is also based on the object of providing a traction drive having the transmission, and a method for controlling the transmission.